[WKB]

Mike,
The tank in the 540 I am building uses a stainless steel marine tank which is about 1mm thick. I used tin snips to cut a hole in it for the fuel pump. This tank is inside the boot with no chance of getting damaged under normal driving conditions. In a cobra it sits at the back of the car exposed to a chance of getting damaged. Aftermarket tanks made of mild steel are usually 2mm thick.
1.6mm SS would be a good choice for strength, but how heavy would it be.
Most cars, before plastic tanks, used metal tanks that would been up to 1.2mm mild steel.
A compromise between strength and weight might be to use 1.2mm SS.

Warren